Bar screens have proven particularly valuable in sorting materials which have unequal dimensions. Wire or punched screens are typically used to sort materials of a granular nature in which all three dimensions are approximately equal. However, many classes of objects, including two of particular commercial interest, wood chips and municipal or industrial trash, are not readily amenable to separation by conventional screening processes.
In the manufacture of paper, logs are reduced to wood chips by chipping mechanisms, and the chips are cooked with chemicals at elevated pressures and temperatures to remove lignin. The chipping mechanisms produce chips which vary considerably in size and shape. For the cooking process, which is known as digesting, it is desirable that the chips supplied have a uniform thickness in order to achieve optimal yield and quality. Ideally, the supplied chips will allow production of a pulp which contains a low percentage of undigested and/or overtreated fibers. Thus, a means is needed to separate chips on the basis of thickness rather than any other dimension. Bar screens have proven particularly adept at separating materials based on a single dimension such as thickness.
With the rise in the recycling culture, a strong demand for an apparatus for separating municipal and industrial trash into its constituent components for recycling has developed. Conventional separation systems which utilize rotating screen drums have proved ineffective. Municipal trash, which typically contains a certain portion of stranded material as well as sheet-like materials, tends to clog the screens. Further, the tumbling action of screens can result in the breakage of components of the municipal waste stream such as glass bottles thereby increasing the difficulty of recycling them.
Bar screens consist of two sets of generally rectangular bars which are joined together. Each set of bars is thus connected into an array or rack. The two sets of bars are interleaved to form a screening bed. The bed consists of the elongated, rectangular bars and the narrow, rectangular spaces between the bars. Material to be sorted is introduced to the surface of the bed and the bars are caused to oscillate so that when one set of bars is going up, the other set is going down.
This oscillatory motion tends to tip wood chips or other relatively small planar objects on edge so that those of a given thickness may slide through the gaps between the bars. Alternatively, it has been found when separating office waste paper, that bar screens prove effective in removing extraneous litter from the recovered office paper.
Recent developments in bar screens have led to using bars of narrower widths. Narrower bars allow greater open area between bars which in turn allows greater screening area for a bar screen of a given size. The thinner bars require bar attachment systems and bar structures which are adapted to provide greater support for the thinner and hence less stiff screening bars. To provide greater stiffness in the rack of thinner bars each bar has, in addition to the two legs which mount the bar to the bar screen, a cantilevered leg which does not attach to any structure. Each cantilevered leg is joined to adjacent legs on adjacent bars by a rod which passes through the cantilevered legs. Spacers constructed of resilient material are placed between the cantilevered legs. The process of assembling the rod, legs, and spacers can be labor intensive.
What is needed is a spacer for positioning between cantilevered legs which facilitates assembly of the screen bars into a rack.